The present invention relates generally to optical information processing, and more particularly to subtracting analog noise from an optical communication channel using stable, non-absorbing optical hard limiters.
In today""s information age, optical communication technologies are being used more and more frequently for transmitting information at very high speeds. As with other types of communication technologies, the accumulation of noise along an optical communication channel degrades the signal-to-noise ratio and thus increases the bit error.
Generally speaking, noise can be removed by filtering out-of-band noise, which would not work when the noise is in the same band as the information, or by filtering in-band noise (U.S. Pat. Nos. 6,044,341, 6,041,126 for voice).
In accordance with one aspect of the invention, analog noise is subtracted from an optical communication channel using stable, non-absorbing optical hard limiters.
In accordance with another aspect of the invention, an optical noise subtractor includes a first optical coupler operably coupled to receive a reference signal B and a control signal of intensity I2 and to output a first combined signal therefrom, a first optical hard limiter operably coupled to receive the first combined signal from the first optical coupler and to output a reflected signal therefrom, a second optical coupler operably coupled to receive an information signal A and a bias signal of intensity I2 and to output a second combined signal therefrom, a third optical coupler operably coupled to receive the reflected signal from the first optical hard limiter and the second combined signal from the second optical coupler and to output a third combined signal therefrom, and a second optical hard limiter operably coupled to receive the third combined signal from the third optical coupler and to output a transmitted signal therefrom. The first combined signal output from the first optical coupler is substantially equal to (B/2+I1). The reflected signal output by the first optical hard limiter is substantially equal to (I1xe2x88x92B/2). The second combined signal output by the second optical coupler is substantially equal to (A/2+I1). The third combined signal output by the third optical coupler is substantially equal to 0.5(I1xe2x88x92B/2+A/2+I1). The transmitted signal output by the second optical hard limiter is substantially equal to 0.5(Axe2x88x92B). I2 is substantially equal to two times I1.
In accordance with yet another aspect of the invention, a method for subtracting analog noise from an optical communication channel using stable, non-absorbing optical hard limiters involves combining a reference signal B with a control signal of intensity I2 through a 3 dB optical coupler to form a first combined signal having an intensity substantially equal to (B/2+I1), processing the first combined signal by an optical hard limiter to form a reflected signal having an intensity substantially equal to (I1xe2x88x92B/2), combining an information signal A with a bias signal of intensity I2 through a 3 dB optical coupler to form a second combined signal, combining the reflected signal and the second combined signal through a 3 dB optical coupler to form a third combined signal having an intensity substantially equal to 0.5(I1xe2x88x92B/2+A/2+I1), and processing the third combined signal by an optical hard limiter to form a transmitted signal having an intensity substantially equal to 0.5(Axe2x88x92B). I2 is substantially equal to two times I1.
In accordance with still another aspect of the invention, a method for subtracting analog noise from an optical communication channel using stable, non-absorbing optical hard limiters involves compressing a reference signal B in the range {0, I2} into a compressed reference signal in the range {I1, I2}, inverting the compressed reference signal to form an inverted compressed reference signal, compressing an information signal A in the range {0, I2} into a compressed information signal in the range {I1, I2}, combining the inverted compressed reference signal and the compressed information signal to form a combined signal essentially subtracting the compressed reference signal from the compressed information signal, and expanding the combined signal in the range {I1, I2} into a transmitted signal in the range {0, I2}, wherein I2 is substantially equal to two times I1.